The present invention relates to a method in a twin-wire drying section of a paper machine which includes upper and lower rows of drying cylinders, an upper drying wire guided by surfaces of the upper drying cylinders and upper guide rolls situated between them and a lower drying wire guided by surfaces of the lower drying cylinders and lower guide rolls situated between them, wherein a web is pressed by the upper wire in direct drying contact with the surfaces of the upper drying cylinders and is pressed by the lower wire in direct drying contact with the surfaces of the lower drying cylinders, the web having a free draw of a certain length between a drying cylinder of one row and a drying cylinder of another row.
The invention also relates to apparatus for use in such twin-wire drying sections and which are adapted to be situated between adjacent drying cylinders in spaces bounded by adjacent drying wire runs and the free sectors of the guide rolls guiding the drying wire.
As used in this application, the term single-wire draw refers to an arrangement wherein a wire runs over heated drying cylinders in a manner such that the web runs from one cylinder row to the other cylinder row while supported by the drying wire so that the web is between the drying wire and the surface of the drying cylinder in one cylinder row while in the other cylinder row the drying wire is situated between the surface of the cylinder and the web. An advantage of the single-wire draw drying section is that the web is always supported by the drying wire and has no free draws, or at least no free draws of any substantial length, which thereby reduces the danger of web breakage.
The term twin-wire draw is used in this application to refer to the well-known arrangement in which a top wire is used in association with a row of top cylinders and a bottom wire is used in association with a row of bottom cylinders, the bottom wire being guided by surfaces of the drying cylinders and by guide rolls positioned between the drying cylinders so that at the top cylinder row the web is pressed by the top wire in direct drying contact with the surfaces of the top cylinders and in the bottom cylinder row the web is pressed by the bottom wire into direct drying contact with the surfaces of the lower cylinders.
The present invention is particularly concerned with methods and apparatus for use in connection with drying sections of the twin-wire draw type.
In twin-wire draw drying sections, the web has a substantially long free draw between the cylinders of one row and the cylinders of the other row. The web tends to flutter over these free draws and there is a posibility of the web breaking or creasing during its run over the free draws. These drawbacks are particularly prevalent at the beginning of the drying section where the web is still relatively wet and therefore weak and sensitive to flutter.
Efforts have been made in the past to eliminate this drawback by shortening the free draws at the beginning of the drying section by positioning the top and bottom rows of drying cylinders closer to each other than they normally would be in the case where optimal drying efficiency would be obtained.
Another approach to overcoming these drawbacks has been to utilize single-wire draw arrangements in the third and fourth drying groups. However, this tends to lower evaporating efficiency and complicates ventilation arrangements, and, therefore is used only in emergency situations.
Efforts have also been made to reduce the flutter of the web by repositioning the wire guide rolls to shorten the unsupported run of the web. A drying group incorporating this feature is disclosed in U.S. Pat. No. 3,753,298. A Swedish paper machine wherein the guide rolls are positioned in accordance with U.S. Pat. No. 3,753,298 has attained speeds of up to 853 m/min as disclosed in an article "Engineering Consideration for Light Weight Paper Drying in High Speed Machines" in the publication Paper Technology and Industry, July/August, 1978. However, problems with web flutter have still been encountered.
The web flutter problem is discussed in a Finnish publication Paperin Valmistus (a textbook and manual III, Part 1 of the Finnish Paper Engineer's Association), pages 699 to 700, wherein it is stated that the flutter of web edges is not usually caused by air flow as is often assumed. If this is the case, the web flutter problem cannot be prevented by controlling air flow in the drying section which has often been attempted.
It is presently the opinion in the industry that web flutter is mainly the result of strong air current flows within the pockets defined within the drying section and by pressure differentials in the pockets as well as in the nips formed by the web, drying wire and cylinder surfaces. The strong air flows and pressure differentials are the consequence of boundary layer flows induced by the moving wire, web and cylinder surfaces.
The pockets mentioned above are formed by the free web draws, free cylinder surfaces, and wires or felts guided by guide rolls. These pockets are closed except at their transverse ends and the ventilation of the pockets is considered to be an important factor from the viewpoint of efficiency and uniformity of the moisture profile obtained.
The running speed of paper machines have been continuously increasing and are now approaching about 1500 m/min. The flutter of the free web draws becomes an even greater problem at such high machine speeds and hampers the runability of the paper machine. Although the passage of a web from the press section to the drying section and supporting the web over a single-wire draw drying section can be controlled with conventional arrangements, difficulties are still encountered in the twin-wire draw drying sections, particularly in the third and fourth dryer groups. Known arrangements are not intended to control the quantity of air being pumped into the pockets of the drying section in a manner so that the web is suctioned into engagement with the drying wire as the wire moves from the cylinder surface into the region of the pocket of the drying section. In particular, in twin-wire draw arrangements, the quantity of air being pumped into a pocket essentially depends on the speed of the machine, the geometry of the transport section and the permeability of the drying wire.
It is also conventional to control the moisture profile in the pockets by dividing blow pipes present in the pockets into transversely extending blocks or sections which can be opened and closed to regulate the quantity of air being blown into the pocket. In this connection, reference is made to Finnish Pat. No. 68,275 of Valmet Oy, assignee of the instant application.